The a historian vs. an historian debate has a pretty straightforward answer: a historian is the correct way to write and say it. “A history” and “a historical” are the best choices, too. This is part of a complete episode.
Careful what you criticize! Not long ago, some words that sound perfectly normal today were considered gauche and grating on the ear. If the complainers had had their way, we couldn’t say a word like pessimism or use contact as a verb! Also...